There is often a requirement for substances that require mixing to form a composition, to be stored separately until use. In some situations it may be necessary to contain small tester amounts or samples in order to permit a trial of the contents before purchase of a larger amount. For some substances, such as hair dyes, cosmetics or topical applications it may be recommended to test a small amount of the substance or composition prior to application or use.
For example there is a large current market for hair dyes, both for professional colouring carried out at hair salons and for private use. Permanent hair dye, containing para-phenylenediamine (PPD), usually consists of two elements that are mixed immediately prior to use: a colourant, and a developer. The hair colouring process can be potentially dangerous causing contact dermatitis, anaphylactic shock and in, albeit rare and extreme cases, death. This is due to the chemicals commonly used in the colourants, developers, or when both are combined. Problems can be exacerbated by the lack of experience and training of the users handling the products.
Most manufacturers recommend that their products are tested on an individual on which they are to be used, typically a day or so before the product is used, each time the product is used. However existing products do not provide a means to carry out a patch test of product without opening the packaging that contains the container(s) and then opening the containers that hold the full amounts. Testing helps to indicate whether the user will or will not have an adverse reaction. Testing every time is recommended as a user's tolerance to the chemicals can change due to the use of drugs (pharmaceutical or recreational), diet, age, tattoos, black henna tattoos, the use of sunscreen and other personal circumstances.
Manufacturers are continuously adjusting the product for marketing, competitive and technical purposes. Test guidelines generally specify that the test should be carried out with the colourant only or a small amount of colourant and developer, depending on the laws of the jurisdiction of use and the particular brand used. Semi-permanent hair dyes also usually require a 48 hour test as some may contain a diluted para-dye, a nitro-dye or an azo-dye within the colourant. Colourant with bleaching booster powders and the developer also recommend a 48 hour test due to the presence of ammonia, hydrogen peroxide and possibly small percentage of the PPD or its derivative.
Allergy UK—an organisation based at www.allergyuk.org—recommend a 48 hour patch test as ‘cross-reactivity’ can occur in a moderate number of patients who have become sensitised to one of the aforementioned chemicals. US Food and Drug Administration also provides guidance on safety issues relating to use of cosmetic products such as hair dye and encourages skin tests before every use—https://www.fda.gov/cosmetics/productsingredients/products/ucm143066.htm
If there is more than one product, or more than one composition, separate tests for each product or composition are recommended.